Interferometric signal analysis involves the determination of signal wavelength, wave velocities, distances and directions using interference phenomena between, e.g., two coherent optical signals. Particular application may include optical signal analysis in a fiber optic gyroscope (FOG).
A FOG typically includes a light source, e.g., a laser diode, which provides coherent light split into two substantially equal waves by a beamsplitter. The beamsplitter is coupled to the ends of a length of optical fiber wound in a coil. The light waves are launched into each end of the coil, and are recombined interferometrically at the coil output such that the light intensity seen by a detector depends on the relative phases of these waves.
When the coil is subject to rotation about an axis normal to the coil, the counter-propagating waves take different times to traverse the coil. This non-reciprocal phenomenon, known as the Sagnac effect, causes a change (shift) in the relative phase between the waves reaching the detector and, therefore, a change in the light intensity signal at the detector. Depending on the initial phase difference, which can be controlled, for example, by application of suitable phase modulation at one end of the coil, the magnitude and sense of the change in the light intensity signal depend, respectively, on the rate and direction of the rotation applied to the coil about the axis.
The rotation-induced change can be compensated for, i.e., nulled, by imposing between the waves a further phase difference, equal and opposite to the Sagnac phase difference. Various methods are known in the art of imposing such a phase difference. For example, a lithium niobate integrated optical phase modulator at one end of the coil can be driven by an appropriate waveform, such as an analog or stepped digital periodic ramp, i.e., a serrodyne waveform. Control of an appropriate waveform parameter, e.g., frequency, in response to rotation alters the modulator-induced phase difference such that it nulls the rotation-induced difference. The value of the parameter that produces the desired null serves as a measure of the rotation rate. For example, in an appropriately initialized FOG employing a fixed-amplitude serrodyne modulation waveform, the serrodyne (ramp) frequency change needed to null the Sagnac phase is proportional to the rotation rate; and the sign, i.e., the direction, of this change indicates the rotation direction.
More specifically, when a serrodyne FOG is not subject to rotation, there are various ramp frequency values, including zero, for which the gyro output is nulled. Frequently, FOG initialization selects "zero" as the operating "setpoint". This value has the advantaqe that it is independent of physical parameters of the FOG. However, the ramp polarity must then reverse whenever the rotation direction reverses, to avoid the need for "negative" frequencies. If the ramp reversal is imperfect, large scale-factor errors may adversely affect low-rate measurements. An alternative to this approach is to select one of the non-zero output-nulling frequencies f.sub.0 as the setpoint. If the FOG is then subject to rotation about an axis normal to the coil, and the ramp frequency is controlled to null the Sagnac phase shift, the difference between the new frequency and the setpoint f.sub.0 serves as a measure of the rotation rate and represents the output of this closed-loop FOG. If the selected value f.sub.0 is sufficiently large, the ramp frequency remains positive even at the largest rotation rates, and rate-dependent ramp polarity reversals are avoided.
However, the non-zero setpoint values typically depend on physical parameters of the FOG, which may be subject to drift, thereby degrading the accuracy in measuring FOG rotation rate. Setpoint drift may be caused by changes in the environment in which the FOG must operate, e.g., changes in temperature, and usually can not be precisely predicted. Unpredictable errors produced by such drift in a FOG operated as described above make it unsuitable for use in systems requiring highly accurate rotation sensing.
Similar arguments are applicable to other closed-loop interferometric sensors in which the phase shift response to a measurand can be nulled by controlling one or more phase modulation parameters in proportion to the sense and magnitude of the measurand.